Low Impact

Engineers in Japan are continuing to develop medium-span bridge solutions that can be built with minimal impact on the environment, and at an efficient cost. Akio Kasuga explains how

Road building in Japan involves the construction of large numbers of tunnels and bridges because of its naturally mountainous terrain. This type of topography can make construction of even modestly-sized bridges a difficult, intrusive and expensive task. In an attempt to try and address these problems, engineers in Japan have been developing a new method of building composite truss bridges using suspension systems. In 2004 (Bd&e issue 37) construction of the Seiun Bridge in Yamashiro demonstrated how the steel truss and deck of a road bridge could be built on suspension cables. Once the deck was complete, the force in the cables, which were initially taken by ground anchors, were tranferred into the concrete upper and lower chords as prestressing forces. By using this construction method, it was possible to build a single-span composite truss without using temporary supports or falsework. Benefits were realised both in terms of construction cost and sustainability, requiring less excavation and minimising the environmental impact of the bridge.

In little more than ten years, this new construction method has progressed from laboratory testing to use in small pedestrian bridges, and from there to use in highway bridges. It has proven to be a superior method for constructing bridges in mountainous regions. The method involves construction of the bridge on top of cables that have been extended across the gap. The horizontal force of the cables anchored through ground anchors in the erection process is transferred to the bridge after completion and acts as prestressing force. This makes it easy to build simple girder bridges of up to 100m length above valleys, without using falsework. These bridges are highly rigid and have sufficient load-bearing capacity to meet requirements.

The first example of this type of construction was the Ganmon Bridge, which was built in 2001. It is a footbridge with deck, inverted V-shaped steel struts, which constitute the upper chord member, and a stress ribbon that constitutes the lower chord member. The bridge has a 37m-long span, and the deck is 3.7m above the stress ribbon at the centre of the span. Almost all of the concrete members were precast using 40MPa concrete. During erection, the precast stress ribbon was suspended from extended cables and launched. The cables were fastened to the abutments, and the horizontal force that was generated was cancelled by ground anchors attached to the abutments. When the upper and lower chord members were complete, the horizontal force of the cables was converted into compressive force to these chord members, forming a self-anchoring structure. The most important aspect of the erection process for this structure was the fact that the ground anchors could be converted to a self-anchoring structure. The stress ribbon was launched with the struts attached, and the deck was launched on rails attached between the struts.

Just a few years later, a similar approach was used for construction of the Seiun Bridge, a highway structure crossing the Yoshino River in Yamashiro-cho on the island of Shikoku. The bridge, which was finished at the end of 2004, is a single-span prestressed concrete composite truss bridge with concrete upper and lower chord members and steel diagonal members.

When the bridge was completed, the girder deck acted as the upper chord member while the stress ribbon acted as the lower chord member.

Use of concrete tension ties for the lower chord member enabled the deflection in the centre to be reduced from 154mm to just 25mm, and the stress variations caused by the live load from the cables to be reduced from 140N/mm² to 20N/mm².

The latest bridge that has been designed and built by this method is the Seishun Bridge. Construction of the earlier structures involved them being built on a suspension system with cables that had been extended first. As a result, the Seiun Bridge, whose upper chord member is heavier than that of the Ganmon Bridge, is most unstable at the point just before the structure is complete. In order to overcome this problem, wire stabilisers were used for the Seiun Bridge, but for the Seishun Bridge, the problem was completely solved by evolving it into a double suspension structure.

The basic concept involves a double row of cables that have been extended. The primary cables are used to support the deck load and the secondary cables are used to adjust the sag of the primary cables. In other words, the bridge constructed not from the bottom up, as in the previous examples, but from the top down. This greatly improves stability during erection. In this method, as in the previous examples, both primary and secondary cables are converted into a self-anchoring system.

Restrictions on the construction of the Seishun Bridge stated that no anchors could remain in the ground when the bridge was completed, and that the maximum gradient of the bridge deck must be no more than 5%. For this reason, a stress ribbon bridge was not suitable, and so a form like that of the Seiun Bridge was chosen.

However, unlike the Seiun Bridge, this structure was a pedestrian bridge and so there was no need to increase the overall rigidity as a truss structure. For this reason, struts were provided and the suspended slab section was made more lightweight through the use of only cable rigidity rather than that of concrete members.

In the construction process, first the primary and secondary cables were extended, and then the precast segments were sent along the primary cables. When the erection of the segments was complete, the struts and temporary struts were put in place and the secondary cables were tensioned to adjust the primary cable sag. Next, the wet joints between the segments were constructed and both primary and secondary cables were transferred to the deck to convert the structure to a self-anchoring one. Finally, the temporary struts were removed to complete the bridge.

The construction process took approximately a month from erection of the precast segments to bridge completion, and the structure was opened last year. The use of the double suspension system greatly improved stability during the erection process.

Composite truss bridges are transparent structures, and the slender upper and lower chord members have structural elegance. Moreover the bridge weight is at first almost entirely supported by the suspension cables, but in the end this is converted into prestressing force applied to the bridge. Accordingly, the method is a rational one that uses no unneccessary materials from construction through to completion, with the exception of the external anchors using during the erection process. The instability during erection that was an initial concern was resolved through the use of a double suspension system.

Akio Kasuga is chief engineer for Sumitomo Mitsui Construction.